The present invention relates to niobium and other valve metals and oxides thereof and more particularly to niobium oxides and methods to at least partially reduce niobium oxide and further to oxygen reduced niobium oxides and other valve metal oxides. The present invention relates to niobium oxides and other valve metal oxides useful, for instance, in the production of capacitors, sintered anode bodies, and the like. The present invention also relates to methods of making niobium oxide particles and other valve metal oxide particles. The present invention also relates to niobium, niobium powder, hydrided forms thereof, and electrolytic capacitors made therefrom. More particularly, the present invention relates to methods of preparing niobium or hydrided niobium feedstock to form suitable niobium powder or hydrided niobium for a variety of uses.
Efforts are continually being made to improve the handling of metal powder, such as niobium powder. In particular, fine powders, for instance, having particle sizes of 0.1-200 microns, can prove difficult to work with and thus, methods to agglomerate or granulate fine metal powder have been developed. In addition to developing methods to agglomerate fine metal powders, efforts have also been made to agglomerate such powders in such a manner that flow properties and/or other desirable properties such as electrical characteristics are maintained or improved.
Development of metal powders suitable for making capacitors has resulted from efforts by both capacitor producers and metal powder processors to delineate the characteristics required for metal powder to best serve in the production of quality capacitors. Such characteristics include surface area, purity, shrinkage, pressability, green strength, flowability, and stability.
Solid state electrolytic capacitors, and valve metal capacitors in particular, have been a major contributor to the miniaturization of electronic circuits. In addition to the reduced size and higher frequencies of current electronic equipment and electronic circuits, a growing demand exists for capacitors offering higher capacitance and lower equivalent series resistance (ESR). Valve metal capacitor anodes typically are manufactured by compressing valve metal powder to less than half of the metal's true density with an anode lead wire to form a pellet, sintering the pellet to form a porous body (i.e., an anode), and then anodizing the porous body by impregnation with a suitable electrolyte to form a continuous dielectric oxide film on the porous body. The anodized porous body is then impregnated with a cathode material to form a uniform cathode coating, connected to a cathode lead wire by soldering, for instance, and encapsulated with a resin casing. Thus, open, uniform pores are important for the steps of anodizing and impregnating the pellet to form the cathode.
Valve metal oxides that have been oxygen reduced, in other words, valve metal suboxides such as niobium monoxide (NbO), TaO, and the like, have been recently recognized as a viable solid electrolytic capacitor material that offers unique performance advantages such as high dielectric stability, low leakage current, and low flammability. These advantages, combined with lower costs, make valve metal suboxides an attractive and economical alternative to valve metal as an electrolytic capacitor material. Conventional methods of preparing niobium suboxide include converting a mixture of stochiometric proportions of niobium pentoxide (Nb2O5) powder and niobium metal powder (acting as an oxygen getter) into NbO via heat treatment under vacuum. However, these methods have the possibility that some incomplete reactions may occur that yield a sub-optimal material such as NbO2 that is interspersed within and difficult to separate from the NbO. Thus, it is advantageous to eliminate the chances of any semi-conducting material being present in the powder material used to form the anodized porous body.
It is hypothesized that the ESR of a capacitor anode is related to the cohesiveness of the primary particles used in formation of the capacitor. The cohesiveness of the primary particles can be related to the quantity and quality of connections between primary particles achieved in agglomerating the primary particles to form agglomerates (agglomerates can be described as clusters of smaller primary particles). Thus, the ESR of a capacitor anode can be decreased via the coarsening that results from thermal agglomeration of the primary powder particles used in forming the anodized porous body. However, coarsening of the particles tends to be accompanied by densification of the particles which reduces the surface area (reduced capacitor capacitance). Thus, the production of agglomerated particles having a large surface area, suitable cohesive strength, and uniform porosity that enable the production of a valve metal capacitor having both high capacitance (i.e., high volumetric efficiency) and low ESR is considerably difficult using capacitor grade powders made by conventional methods.
In addition, conventional methods of preparing a valve metal suboxide powder typically produce powder particles having a relatively rough, irregular surface texture. The rough particle surface tends to retain significantly more organic material during formation of capacitors from the valve metal suboxide powder. Residual organics can result in high levels of residual carbon in the finished capacitors, causing high DC leakage of the capacitors.
With an ever increasing demand for capacitor materials such as tantalum, alternatives to tantalum have become an important priority in order to meet industry demands. Niobium is becoming one of the alternatives to tantalum but as the industry has realized, niobium is not a complete substitute for tantalum due to niobium not providing the same electrical properties. Accordingly, further developments in the niobium area continue today.
Another alternative to tantalum is niobium metal oxides that have been oxygen reduced, in other words, niobium sub-oxides such as NbO and the like. The oxygen reduced niobium oxides show considerable promise as providing an additional material that can be used in the formation of capacitor anodes. In order to further satisfy industry demands, several properties of the oxygen reduced niobium oxides can preferably be improved such as the crush strength of the oxygen reduced niobium oxides as well as efforts to reduce the amounts of contamination that occurs in the manufacturing of the oxygen reduced niobium oxides. In addition, acid leaching is commonly used to reduce the level of contamination occurring when niobium is milled to achieve particular particle sizes. The acid leaching step complicates the manufacturing process and leads to the manufacturing process being more expensive. In addition, the flow property of the oxygen reduced niobium oxides could be further improved to better satisfy industry standards.
While there are various methods to make oxygen reduced niobium oxides and other oxygen reduced valve metal oxides, there is a constant need to improve upon the resulting properties of the final product. In some processes, the treatment steps can cause a loss of surface area and other treatment steps can cause a loss of flow properties and a decline in other favorable properties generally useful in the fabrication of capacitor anodes. For instance, the sintered crush strength of the oxygen reduced niobium oxide powders is a desirable property which can be generally low with current oxygen reduced niobium oxides. However, in any effort to improve upon this property, other properties can be affected such as the surface area, pore structure, pore size distribution, flow properties, and the like. Thus, there is a need in the industry to provide a method, as well as to provide products which provide a fine balance of properties, including a desirable sintered crush strength and green strength, along with other properties, such as BET surface area, capacitance capability, particle size, and the like.
The metal powder should provide an adequate surface area when formed into a porous body and sintered. The CV/g of metal capacitors can be related to the specific surface area of the sintered porous body produced by sintering a metal powder pellet. The specific surface area of metal powder can contribute to the maximum CV/g attainable in the sintered porous body.
Purity of the powder can be an important consideration as well. Metallic and non-metallic contamination tends to degrade the dielectric oxide film in metal capacitors. While high sintering temperatures serve to remove some volatile contaminants, high temperatures tend to shrink the porous body reducing its net specific surface area and thus the capacitance of the resulting capacitor. Minimizing the loss of specific surface area under sintering conditions, i.e., shrinkage, is helpful in producing high CV/g metal capacitors.
Flowability of the metal powder and green strength (mechanical strength of pressed unsintered powder pellets) are also important characteristics for the capacitor producer in order to provide efficient production. The flowability of the agglomerated metal powder can be important to proper operation of automatic pellet presses. Sufficient green strength permits better handling and transport of a pressed product, e.g., pellet, without excessive breakage.
A “pellet,” as the term is used herein, is a porous mass or body comprised of metal particles or oxides thereof. Green strength is a measure of a pellet's unsintered mechanical strength. The term “pressability” describes the ability of a metal powder to be pressed into a pellet. Metal powder or oxides thereof that forms pellets that retain their shape and have sufficient green strength to withstand ordinary processing/manufacturing conditions without significant breakage have good pressability.
A desirable characteristic of metal powders or oxides thereof of relatively fine particle size is stability. Stability of metal powders can be achieved by surface passivation of the particles with, for example, oxygen or an oxide layer. Surface passivation is typically accomplished in a separate passivation step.
Accordingly, a need exists to provide fine metal particles such as niobium powder, not only to address the problems of fine powders but also to lead to agglomerated metal particles that have desirable properties such as good flow properties and improved pore size distribution.
Ongoing efforts persist to develop superior niobium materials and to refine niobium preparation processes to produce capacitor grade metal material that can be formed into high performance capacitors characterized by high capacitance (CV/g) and low DC leakage. Examples of morphology and other observable or measurable microstructure characteristics of capacitor grade material that can affect the performance characteristics of capacitors made therefrom, include primary particle size (D50), granule size, flow, purity, degree of roundness, specific (BET) surface area, particle size distribution (e.g., D10, D50, and D90), Scott density, pressability, crush strength, porosity, stability, dopant content, alloy content, and the like.
Niobium metal oxides that have been oxygen reduced, in other words, niobium suboxides such as niobium monoxide (NbO) and the like, have been recently recognized as a viable solid electrolytic capacitor material that offers unique performance advantages such as high dielectric stability, low leakage current, and low flammability. These advantages, combined with lower costs, make niobium suboxides an attractive and economical alternative to tantalum as an electrolytic capacitor material. Conventional methods of preparing niobium suboxides typically include converting a mixture of stochiometric proportions of Nb2O5 powder and niobium metal which acts as an oxygen getter into niobium suboxide(s) via heat treatment under vacuum. In addition to being relatively expensive due to the historically high cost of niobium metal, reduction methods using niobium metal offer significant challenges in controlling morphology and microstructure of the oxygen reduced final product (e.g., NbO) to obtain performance characteristics, such as high CV/g.
In addition to limited control over final product morphology, conventional methods of preparing niobium suboxide that use solid getter materials other than niobium metal, such as tantalum and magnesium, have other drawbacks. For instance, the final niobium suboxide product can become contaminated in the reduction process by unreacted or residual getter material and/or oxidized getter material being mixed in with the niobium suboxide. The likelihood of contamination is increased when the getter material is physically contacted with the starting niobium oxide such as by homogenizing, blending, mixing or the like. Also, getter materials used in conventional reduction reactions typically have high atomic weight and particles that provide a relatively low surface area with which the oxygen can react. Thus, currently used getter materials must be present in large quantities to reduce a given amount of starting niobium oxide. As a result, the extent of contamination of the reduced oxygen niobium oxide is greater because the ratio of getter material to the niobium oxide is high. Preparation of the final product by decontaminating the niobium suboxide by screening or acid leaching the niobium suboxide, for example, becomes more difficult and more waste is produced.
Accordingly, a need exists for an oxygen reduced niobium oxide such as NbO that is less expensive to produce than niobium suboxides produced by current methods that involve using niobium metal to reduce niobium oxides. A further need exists for a process that provides a greater degree of control over the morphology, microstructure, and/or particle size distribution of the oxygen reduced final product than what is presently possible using conventional reduction methods. A need also exists for a method to reduce a niobium oxide in which contamination of the oxygen reduced niobium oxide during the reaction process is minimized by having a comparatively low ratio of getter material to niobium oxide relative to conventional methods. Also needed are capacitors that have superior performance characteristics such as high capacitance and low DC leakage made from niobium suboxides having superior morphology.
A need also exists for a method of making primary and agglomerated valve metal oxide particles useful in producing a valve metal sintered body, that suitably controls the surface area, the cohesive strength, the porosity, the crush strength, and other properties of the valve metal oxide particles used as the capacitor-grade material, as well as minimizes the presence of semi-conducting valve metal oxides in the capacitor-grade material.
Accordingly, a need exists to overcome one or more of the above-described disadvantages.